Current divider and ramp generator with relatively long time constant using such a current divider

Abstract: A circuit for dividing a reference current (I r ) is composed by an n number of transistors (Q₁,Q₂,Q₃) connected in cascade, in a Darlington configuration, between current generator and a fractionary current output node (V₂) and by N+k, where k is an integer different from zero, directly biased diodes (D₁-D₄) in series, connected between the generator and said fractionary current output node. The circuit does not employ current mirrors and conveniently all transistors may have the minimum size of the process, which also minimizes the effects of leakage currents. Additionally, means may be used for compensating the leakage currents from the tubs of the transistors. The circuit is useful as a capacitance multiplier, or as a slow ramp generator in a large number of design situations. Independency from intrinsic parameters of the transistors used and/or from temperature of operation may be provided by employing a specifically designed reference current generating means. Several embodiments are described.

Description: The present invention relates to a current dividing circuit suitable for use in an integrated capacitance multiplier circuit, that is to a ramp generator with a relatively long time constant that may be entirely integrated in a monolithic form. The circuit is able to provide a time constant in the order of several hundreds milliseconds, without requiring large capacitances that would be difficult to integrate and is useful in a large number of applications. An ever increasing number of functions that are implemented in an integrated circuit (with the consequent limitation of the number of pins available) has led to the development of circuits that require a minimum number of externally connected components. Notably parameters or functions that may be entirely integrated with increasing difficulty are relatively large time constants, for example in the order of several hundreds milliseconds, which normally entail the use of large capacitances that are hardly integratable. So-called capacitance multiplier circuits that can be fully integrated have been known for a long time. They commonly employ an operational amplifier, capable of controlling the charge current, necessarily of an extremely small value, of an integrated capacitor. A capacitance multiplier circuit, having improved characteristics of precision and termal drift, is described in the European patent application No. 92830419.5, filed on July 28, 1992, by the present Applicant and published as EP-A-0 580 920. Capacitance multiplier circuits based on an operational amplifier are relatively complex and require a non-negligeable area of integration. A similar function may also be implemented by employing a simple constant current generator and a circuit capable of dividing the current by a factor in the order of thousands. Such circuits may be implemented by employing a plurality of cascaded current dividers, realized by current mirrors employing transistors of different size. A circuit of this kind, capable of achieving a ramp slope of about 100V/sec with an integrated capacitance (C) of 10pF, employing an easily integratable current generator capable of delivering a current of about 10µA, is shown in Fig. 1. In the reported example, it is necessary to divide the current Ir by a factor equal to 10.000. As shown in Fig. 1, the circuit may employ four cascaded current mirrors (current dividers), each dividing the current by 10. A circuit of this type, is less burdensome than a capacitance multiplier based on the use of an operational amplifier, but still requires a non-negligeable area of integration and the leakage currents of the tubs within which the large size transistors are found and the substrate of the integrated circuit may assume a value that is comparable with the current I2 (of about 1nA) that must be delivered to the integrated capacitance C. At high temperature, the capacitance C may not charge at all. Also an equivalent circuit of that depicted in Fig. 1, but implemented with NPN transistors instead than with PNP transistors would not provide any significant improvement. The main aim of the present invention is to provide a current dividing circuit usable in a ramp generator or a capacitance multiplier that can be fully integrated in a monolithic form while employing a reduced number of transistors that may have the minimum process size. It is a further object of invention to provide a slow ramp generator or capacitance multiplier having reduced area requirements and provided with means for compensating the leakage current of the tubs wherein the transistors that compose the circuit are formed, in order to ensure reliability of the circuit also at a high temperature of operation. Basically, the circuit of the invention rests on the principle of employing the voltage drop accross a number N of directly biased junctions for biasing a number N+k of junctions, where k an integer different from 0, and employing the current flowing through said N+k junctions for charging an integrated capacitance. The N directly biased junctions are realized by an N number of transistors connected in cascade in a Darlington configuration, functionally connected between a reference current generator and a capacitor to be charged through a series of N+k diodes (junctions). The diodes are directly biased and functionally connected between the same reference current generator and the capacitor, schematically in a parallel arrangement with the cascaded Darlington configured transistors. All transistors and diodes (or diode configured transistors) may have the minimum size that is allowed by the fabrication process of the integrated circuit. It may be shown that the charge current of the integrated capacitance, which flows through the series of diodes, is inversely proportional by a factor given by the current transfer ratio (h FE ) of the N transistors used, in a common-emitter configuration, elevated at a power given by the ratio N/N+k (h FE N/N+k ). Using an integrated capacitance of about 10pF, ramp slopes of about 100V/se may be easily achieved with only three, Darlington configured cascaded transistors and four diodes (diode- configured transistors). The charge current of the integrated capacitance that flows through the four diodes is given, in first approximation, by the product of the current generated by the current generator, elevated at a power 3/4, with the saturation current (Is) of the transistors used at a power of 1/4, divided by h FE 3/4 . The circuit of the invention may also be implemented in different ways, for example in a way suitable to untie the value of the charge current from the h FE parameter of the transistors used and/or compensate for non-negligeable leakage currents, at a high temperature of operation. The different aspects and advantages of the invention will become more evident through the following description of several important embodiments and by referring to the attached drawings, wherein: Figure 1 shows an integrated capacitance multiplier that may be employed as a ramp generator with a relatively large time constant, according to a known practice, as described above; Figure 2 shows a circuit according to the present invention which is functionally equivalent to the circuit of Fig. 1; Figure 3 shows the circuit of Fig. 2 provided with means for compensating leakage currents; Figure 4 shows a circuit made according to an alternative embodiment of the invention; Figure 5 shows an embodiment of a reference current generator employed in the circuit of Fig. 4. Figure 2 shows an integrated capacitance multiplier circuit that can be used as a ramp generator with a relatively large time constant, made according to the present invention. According to this first embodiment, the circuit employs three bipolar PNP transistors (Q1, Q2 and Q3), connected in cascade, in a Darlington configuration (common emitter). The base of Q3 is connected to the integrated capacitor C, the emitter of Q1 is connected to a reference current generator Ir, while the collectors of the transistors are connected in common to a common ground potential. Four diodes (PN junctions), D1, D2, D3 and D4, are directly biased and are connected in series between the reference current generator Ir and the integrated capacitance C. Consistently with the general requirements of minimizing the integration area, all the transistors used: Q1, Q2 and Q3, as well as the four diode configured transistors: D1, D2, D3 and D4, may avantageously have the same minimum dimensions that are permitted by the fabrication process of the integrated circuit. The voltage drop across the three directly biased PN junctions (emitter-base junctions) of the transistors Q1, Q2, Q3, that is V1-V2, determines the biasing of the four PN junctions, that is of the diodes D1,D2, D3 and D4, thus controlling the current I2 that flows through the diode chain D1...D4, and which charges the integrated capacitance C (neglecting the contribution of the base current of the transistor Q3). By analizing the circuit one may write: where Vt represents the so-called "thermal voltage" (equivalent to KT/q), Is represents the saturation current of the transistors, β 1 and β 2 the current gains of Q1 and Q2, respectively (in a common emitter configuration). From the equation (1) one obtains: Therefore, it is possible to derive the current I2 that flows through the four diodes in series: Bringing the term 1/4 to exponent, neglecting unity and simplifying, the equation (3) may be simplified as follows: I 2 = I s 4 I 3 1 β 2 1 β 2 I 3 s that is: I 2 = I ¾ 1 I ¼ s β ½ 1 β ¼ 2 In practicing the invention with a common fabrication process, the decoupling transistor Q1 may be a PNP transistor of minimum area, as allowed by the process, for example of about 4300 µm 2 . If the base current of Q1 may be neglected and I r =I 1 , by assuming: I1=30µA, Is= -16 A, β 1 =β 2 =100, the current I2 that is delivered by circuit will be equal to 1,28nA, which is over 20.000 times less than I r . By assuming I r =I 1 and considering negligeable the base current of Q1, the expression (5) may be rewritten as: I 2 = I r ¾ I s ¼ h FE ¾ Substantially, starting from a reference current I r , it is possible to derive a fractionary current I 2 , in a ratio that may be of four orders of magnitude, while employing a limited number of transistors, all of minimum size , with a marked saving of integration area. The fractionary current I 2 , controlled by the voltage difference V 1 -V 2 , produced by the circuit of the invention, may be usefully employed for charging an integrated capacitance C, in order to produce on an output mode of the circuit ( V out =V 1 ), a voltage ramp. The expression of the slope of the generated ramp may be obtained by dividing the expression (6) by the capacitance C, having assumed: I r =I 1 and dV 1 /dt=dV 2 /dt. This derivation is justified by the fact that the voltage V 1 -V 2 is constant because a constant current flows through the transistors Q1, Q2 and Q3. Therefore the slope of the ramp will be given by: dV 1 dt = I 2 C = I r ¾ I s ¼ Ch FE ¾ For a capacitance value of 10pF and a reference current I r of about 10µA, ramps with a slope of about 100V/sec may be obtained. It is evident that the circuit of the invention lends itself to implement a capacitance multiplier circuit, that is a ramp generator circuit having a relatively large time constant, which is useful in a large number of design situations. The embodiment depicted in Fig. 2 essentially employs bipolar PNP transistors. Of course the circuit may also be implemented with bipolar NPN transistors. In an embodiment as the one depicted in Fig. 2, employing PNP transistors, the leakage currents of the tubs wherein the transistors are formed and the substrate of the integrated circuit could introduce errors and jeopardize the correct functioning at high temperature. This occurence may be prevented by implementing the circuit as shown in Fig. 3. The leakage of current of each tub (corresponding to the base region of a transistor), may be effectively "recovered" (or compensated for) by employing an auxiliary PNP transistor, having a collector split in two parts. A first portion of the collector is short-circuited to the base to implement a diode configuration of the transistor, while the other portion of the collector injects in the tub of the transistor (that is in the base region thereof) a current that compensates the leakage current towards the substrate. Of course, also these auxiliary transistors that may be employed for compensating for the leakage current, may be transistors of minimum size, similar to the size of the other transistors that form the functional circuit. According to an alternative embodiment of the invention, in order to satisfy particular design requirements of the circuit, for example in order to ensure a ramp slope independent from the h FE parameters of the transistors used, the circuit may be implemented in an alternative form, as shown in Fig. 4. The value of the charge current I 2 that is delivered to the integrated capacitance C may be untied from a direct dependency from the current transfer ratio (h FE ) of the transistors in a common emitted configuration, by employing as a reference current I r a current that is purposely made dependent from the same parameters of the transistors h FE . This is obtained by generating the reference current I r by means of a transistor Q4, driven through its space by a current generator I r ', which is independent of the current gain of the transistors. In this way it is possible to nullify the dependency of the slope of the generated voltage ramp from said intrinsic parameter of the transistors used. With reference to the circuit of Fig. 4, it is possible to demonstrate that the slope of the ramp is given by: dV 2 dt = I 2 C = I r ¾ I s ¼ C A particularly preferred embodiment of a generator of a current I r ', independent of the gain of transistors used, may be based on the use of a current generator that depends from the reverse junction current of the transistors used (substantially equivalent to the saturation current I s of the transistors). Moreover, the generator may also be designed in such a way as to compensate, in terms of temperature drift, the ramp generating circuit. A current generator that satisfies both these requirements is depicted in Fig. 5. The reference current generator I r ' of Fig. 5 is dependent on the reverse junction current (which is substantially equivalent to the saturation current I s ) of the transistor Q A according to the following relationship: I r ' = KI s -1/3 where K is a constant. Moreover, the current generator of Fig. 5 is made temperature dependent according to an exponential law. It may be shown that the exponential factor of dependency from temperature may be pre-established by fixing the ratio between the resistences R1 and R2. In the embodiment shown in Fig. 4, an effective temperature compensation of the entire circuit is achieved by verifying the following condition: R 1 R 2 = 5 4

Claims: A circuit for dividing a reference current (Ir) generated by a current generator, characterized by comprising a number N of bipolar transistors (Q1,Q2,Q3) in a Darlington configuration having N directly biased junctions connected in cascade between the output of said current generator and a fractionary current output node; a number N+k, where k is an integer different from zero, of series-connected, directly biased diodes (D1-D4), connected between said output of said current generator and said fractionary current output node. A circuit as defined in claim 1, wherein each diode is constituted by a diode-configured transistor and all transistors composing the circuit have the minimum process size. Circuit as defined in claim 1, wherein all transistors are bipolar PNP transistors. A circuit as defined in claim 3, wherein said N PNP transistors are realized in separate tubs of a semiconducting substrate of an integrated circuit and wherein the leakage current of each tub, constituting a base region of the respective transistor towards the substrate is compensated by means of an auxiliary PNP transistor. A circuit as defined in claim 4, wherein each leakage current compensating PNP transistor has a collector split in two parts, the first part being short-circuited with the base of the transistor, the other part of the collector being connected to the base of a relative PNP transistor and the emitter is connected to a supply rail (V cc ) of the circuit. A circuit as defined in claim 1, wherein said reference current generator is a transistor (Q4) driven through its base by a current generator having a predetermined dependence characteristic on one or more parameters to implement a compensation of drifts of said parameters. A circuit as defined in claim 6, wherein said current generator generates a driving current (Ir') proportional to the current transfer ratio of the transistors in a common emitter configuration. A circuit as defined in claim 6, wherein the said generator generates a driving current dependent according to an exponential law relationship on the temperature of operation. A ramp generating circuit with a relatively long time constant, monolithically integratable, which comprises at least a reference current generator (Ir), an integrated capacitor (C) and means for dividing the reference current to charge said integrated capacitor with a fractionary current of said reference current, characterized in that said reference current dividing means is a circuit according to one of claims 1 to 8.

Inventors: BOTTI EDOARDO, CHIOZZI GIORGIO, BOTTI, EDOARDO, CHIOZZI, GIORGIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H03K4
IPC codes: H03H11, H03K4, H01L21, H01L27